Susceptibility-induced contrasts resulting from marker materials, such as paramagnetic materials, have gained increasing interest in MRI due to their far reaching applications. These applications include, but are not limited to, cell-tracking, molecular imaging or, in interventional MRI, guidance of devices marked by, e.g., paramagnetic materials. As a result, many different methods have been proposed to generate susceptibility-related NMR image contrast. Generally, these methods are either based on negative (signal loss) or positive (signal gain) contrasts, that is, MR imaging sequences that either decrease or increase signals at susceptibility-related field distortions over background signals, that is, signals in the far-field of the magnetic field perturber.
Local magnetic field distortions, however, do not only arise from marker materials but also from susceptibility differences between tissues (e.g. air-cavities, bone tissue interface, etc.), that is, in regions that are typically far from the iso-center of the main magnetic field. Thus, discrimination of susceptibility-related contrast signals resulting from marker materials (wanted) and from other materials in the background (unwanted) is a challenging task. State-of-the-art methods often suffer from unwanted background signal enhancements that limit their practical use for MR-guided clinical interventions, cell tracking and/or molecular imaging. In particular, those unwanted background signal enhancements lead to false-positives what may have a considerable negative impact on patient safety.
In view of the above, it is apparent that there exists a need in the art for imaging methods and/or apparatuses which solve or at least ameliorate the above drawback of background signal discrimination of the prior art. It is a purpose of this invention to fulfill this need in the art as well as other needs which will become more apparent to the skilled artisan once given the following disclosure.